Polymerization of a Cysteinyl Peptidolipid Langmuir Film Jianmin Xu, ² Changqing Li, ² Chengshan Wang, ² Jinhai Wang, ‡ Qun Huo, ‡ and Roger M. Leblanc* ,² Department of Chemistry, UniVersity of Miami, 1301 Memorial DriVe, Coral Gables, Florida 33124, and Nanoscience Technology Center and Department of Chemistry, UniVersity of Central Florida, 12424 Research Parkway, Suite 400, Orlando, Florida 32826 ReceiVed October 13, 2005. In Final Form: October 19, 2005 The surface pressure-area isotherm of a cysteinyl peptidolipid on a pure water subphase (pH 5.8) was compared with that on a water subphase saturated with oxygen and buffered with ammonium bicarbonate (pH 7.8). A reduction of the limiting molecular area was observed for the isotherm measured on the subphase saturated with oxygen. Hysteresis in the compression-decompression cycles of the Langmuir film was also observed. Taking into consideration the chemical structure of the peptidolipid, we rationalized that the free sulfhydryl groups of the peptidolipid were oxidized in the presence of oxygen in the alkaline subphase to form intermolecular disulfide bonds at the air-water interface. The surface topography of the peptidolipid Langmuir film was observed by epi-fluorescence microscopy and the Langmuir-Blodgett film by environmental scanning electron microscopy (ESEM). The micrographs showed evidence of the polymerization of the cysteinyl peptidolipid at the air-water interface. Furthermore, the XPS spectra of the Langmuir-Blodgett films also proved the existence of disulfide bonds. The control peptidolipid C 18 -Ser- Gly-Ser-OH showed identical surface pressure-area isotherms in the presence or absence of an oxygen-saturated subphase. Introduction A Langmuir film at the air-water interface represents a thin film technique that has broad applications in many important disciplines such as chemo- and biosensor developments, 1,2 building blocks for nanomaterials, 3 and a model system for mimicking biointerfaces and their functions. 4 At air-solid interfaces, a Langmuir-Blodgett (LB) film and a self-assembled monolayer (SAM) are both thin film techniques that have been well studied. 5-8 LB films are physically adsorbed onto the solid substrate surface, while self-assembled monolayers are covalently bonded to the substrate material. 9 On the other hand, Langmuir film formation is a dynamic process in which the monolayer structure and property could be carefully controlled during the compression stage, while both the LB and SAM methods lack this useful feature. LB and SAM films are complementary tools in thin film-based research and may be used to target different areas of research. To overcome the limitation of the low stability of Langmuir films, cross-linking of the film could provide an attractive solution. 10-12 Langmuir films could be polymerized through the photopolymerization of diacetylene groups 13,14 or the sol-gel process at the air-water interface. 15 The stability of these polymerized films was greatly improved due to the robustness of the polymer backbone. Huo et al. 16 started a novel research direction by using peptidolipids to simulate specific artificial protein structure. During the compression of the peptidolipid Langmuir film, the peptide moieties organize into supramolecular assemblies with proteinlike structures. These artificial protein structures may be used to develop biomimetic sensors and model systems for studying interactions at biointerfaces. Since the proteinlike supramolecular assemblies are formed through noncovalent bonding, the stability of these assemblies needs to be improved to broaden their application. Unfortunately, the two above- mentioned polymerization approaches are not suitable for the cross-linking of a peptidolipid Langmuir film: diacetylene groups on the hydrocarbon chains, via photopolymerization, may interact with the aromatic amino acids such as tryptophan, tyrosine, and phenylalanine. The sol-gel process proceeds through proton catalysis in acidic media; under that condition, a peptidolipid Langmuir film could have an effect on the supramolecular arrangement through hydrogen bonding interactions between the peptidolipids. Considering the fact that most protein tertiary structures are stabilized with disulfide bonds between cysteine residues, 17,18 we rationalize that disulfide bond formation may be an appropriate way to cross-link the peptidolipid Langmuir film, and therefore to increase the stability of the film. Disulfide bonds could be formed from cysteine residues in many different ways. 19,20 S-Protected cysteines could be oxidized to form disulfide bonds by I 2 in various solvents such as acetic * To whom correspondence should be addressed. Telephone: (305) 284- 2194. Fax: (305) 284-6367. E-mail: rml@miami.edu. ² University of Miami. ‡ University of Central Florida. (1) Kele, P.; Orbulescu, J.; Calhoun, T. L.; Gawley, R. E.; Leblanc, R. M. Langmuir 2002, 18, 8523. (2) Cheek, B. J.; Steel, A. B.; Miller, C. J. Langmuir 2000, 16, 10334. (3) Khomutov, G. B.; Gubin, S. P.; Khanin, V. V.; Koksharov, A. Y.; Obydenov, A. Y.; Shorokhov, V. V.; Soldatov, E. S.; Trifonov, A. S. Colloids Surf., A 2002, 198, 593. (4) deKruijff, B. Nature 1997, 386, 129. (5) Schwartz, D. K. Surf. Sci. Rep. 1997, 27, 245. (6) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir- Blodgett to Self-Assembly; Academic Press: New York, 1991; p 278. (7) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. 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Res. 1995, 8, 321. 181 Langmuir 2006, 22, 181-186 10.1021/la0527700 CCC: $33.50 © 2006 American Chemical Society Published on Web 11/24/2005